One example of known output-voltage control devices is a switching regulator that calculates a logical NAND of an output signal of an oscillator (OSC) and an output signal of a PWM (pulse width modulation) control circuit, or a PWM controller, to forcibly produce an OFF period in an ON period of a driver for driving a switching transistor.
In such a know output-voltage control device, however, the ON period of the driver is dependent on, for example, the response time of a current-controlling element, such as a comparator, provided in the PWM control circuit. Thus, in principle, the ON period of the driver does not become shorter than the response time. Typically, step-up DC-DC converters satisfy:
                              Vin          Vout                =                              toff            T                    =                                    T              -              ton                        T                                              (        1        )            where Vin indicates is an input voltage Vin, Vout indicates an output voltage, T indicates one cycle period, toff indicates the OFF period of the driver, and ton indicates the ON period of the driver.
According to expression (1), when a desired output voltage Vout is to be obtained at a given switching frequency, a reduction in the ON period ton is a requirement for increasing the range of the input voltage Vin that allows for control of the output voltage Vout. In addition, when a desired output voltage is to be obtained with a desired input-voltage range, the switching frequency is limited by a minimum ON period. These problems also apply to step-down DC-DC converters. Step-up/step-down DC-DC converters also have problems in that the output voltage fluctuates greatly during mode switching, and as such, the operating efficiency in a step-up/step-down mode decreases.